LDAF1 and Seipin Form a Lipid Droplet Assembly Complex

Phosphatidic acid drives spatiotemporal distribution of Pex30 at ER-LD contact sites

Lipid droplets (LDs) are ubiquitous neutral lipid storage organelles that form at discrete subdomains in the ER bilayer. The assembly of these ER subdomains and the mechanism by which proteins are recruited to them is poorly understood. Here, we investigate the spatiotemporal distribution of Pex30 at the ER-LD membrane contact sites (MCSs). Pex30, an ER membrane-shaping protein, has a reticulon homology domain, a dysferlin (DysF) domain, and a Duf4196 domain. Deletion of SEI1, which codes for seipin, a highly conserved protein required for LD biogenesis, results in accumulation of Pex30 and phosphatidic acid (PA) at ER-LD contact sites. We show that PA recruits Pex30 at ER subdomains by binding to the DysF domain. The distribution of Pex30 as well as PA is also affected by phosphatidylcholine (PC) levels. We propose that PA regulates the spatiotemporal distribution of Pex30 at ER subdomains that plays a critical role in driving the formation of LDs in the ER membrane.

Lipid droplet - organelle crosstalk and its implication in cancer

Lipid droplets (LDs) store lipids in cells, provide phospholipids for membrane synthesis, and maintain the intracellular balance of energy and lipid metabolism. Undoubtedly, the crosstalk between LDs and other organelles is the foundation for performing functions. Many studies indicate that LDs promote tumor progression. LD accumulation has been observed in a variety of cancers, and high LD content is associated with malignant phenotype and poor prognosis of cancers. In this paper, we summarized the intimate crosstalk between LDs and intracellular organelles, including endoplasmic reticulum (ER), mitochondria, lysosomes and peroxisomes, and addressed the effects of LD-organelle crosstalk on cancer initiation and progression. We also integrated the changes of LD-organelle interactions in cancers to provide an insightful knowledge for cancer therapeutics.

Nuclear receptor signaling regulates compartmentalized phosphatidylcholine remodeling to facilitate thermosensitive lipid droplet fusion

Lipid droplet (LD) fusion plays a key role in cellular fat storage. How the phospholipid monolayer membrane of LD functions in fusion, however, is poorly understood. In Caenorhabditis elegans, loss of cytochrome P450 protein CYP-37A1 causes de-repression of nuclear receptor DAF-12, promoting thermosensitive LD fusion. Here, we report that in cyp-37A1 mutants, DAF-12 up-regulates the transcription and LD localization of seven fatty acid desaturases (FAT-1 to FAT-7) and a lysophosphatidylcholine acyltransferase 3 (LPCAT3) homolog MBOA-6. LD-targeting of these enzymes increases phosphatidylcholine (PC) containing ω-3 C20 polyunsaturated fatty acids, which are essential for thermosensitive fusion. ω-3 C20-PC increase LD membrane fluidity, as does high ambient temperature. Lowering LD membrane fluidity by a chemical membrane rigidifier attenuates thermosensitive fusion; ectopic targeting of ω3 desaturase FAT-1 or MBOA-6 to LDs increases fusion kinetics and thermosensitivity. Furthermore, human LPCAT3 localizes to LDs, positively regulates LD size in human cells and facilitates thermosensitive fusion in C. elegans. These results demonstrate that DAF-12 signaling regulates compartmentalized membrane remodeling and fluidization to facilitate conserved thermosensitive LD fusion.

Incorporation of Triacylglycerol and Cholesteryl Ester Droplets in Phase-Separated Giant Unilamellar Vesicles

Cytoplasmic lipid droplets form from the endoplasmic reticulum (ER). Because the ER membrane can undergo phase separation, the interaction of lipid droplets with phase-separated bilayers is of significant interest. In this study, we used fluorescence microscopy to investigate the incorporation of droplets composed of triolein, trilinolein, trimyristolein, trieicosenoin, and cholesteryl arachidonate in the bilayers of giant unilamellar vesicles (GUVs) consisting of a mixture of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and cholesterol. After the triacylglycerol droplets were incorporated, the DOPC/DPPC/cholesterol (3:3:2) GUVs, which exhibited liquid-disordered (Ld) and liquid-ordered (Lo) phase separation, retained their phase-separated state. The triacylglycerol droplets were predominantly partitioned in the Ld domains. To elucidate the basis of this preferential partitioning, we investigated the surface pressures of DOPC, DPPC, and cholesterol monolayers containing triolein at the air/water interface using a Langmuir trough. From these measurements, we determined the interfacial tension at the monolayer-covered triolein/water interface. The results showed that DOPC most effectively reduced the interfacial tension. Thus, the droplet sorting into the DOPC-enriched Ld domains likely arose from the difference in the abilities of the two phases to stabilize the droplet interface. In contrast, cholesteryl arachidonate had a profound effect on bilayer phase behavior. Fluorescence images of the DOPC/DPPC/cholesterol (3:3:2) GUVs showed that the domain structures disappeared after droplet incorporation. Additionally, surface pressure measurements of DOPC/DPPC/cholesterol (3:3:2) monolayers containing cholesteryl arachidonate at the air/water interface suggested that cholesteryl arachidonate weakened the lipid-lipid interaction. The results indicate that the cholesteryl arachidonate molecules diffused across the bilayer to hinder the bilayer phase separation.

Lipid droplets in the nervous system: involvement in cell metabolic homeostasis

Lipid droplets serve as primary storage organelles for neutral lipids in neurons, glial cells, and other cells in the nervous system. Lipid droplet formation begins with the synthesis of neutral lipids in the endoplasmic reticulum. Previously, lipid droplets were recognized for their role in maintaining lipid metabolism and energy homeostasis; however, recent research has shown that lipid droplets are highly adaptive organelles with diverse functions in the nervous system. In addition to their role in regulating cell metabolism, lipid droplets play a protective role in various cellular stress responses. Furthermore, lipid droplets exhibit specific functions in neurons and glial cells. Dysregulation of lipid droplet formation leads to cellular dysfunction, metabolic abnormalities, and nervous system diseases. This review aims to provide an overview of the role of lipid droplets in the nervous system, covering topics such as biogenesis, cellular specificity, and functions. Additionally, it will explore the association between lipid droplets and neurodegenerative disorders. Understanding the involvement of lipid droplets in cell metabolic homeostasis related to the nervous system is crucial to determine the underlying causes and in exploring potential therapeutic approaches for these diseases.

Targeting membrane contact sites to mediate lipid dynamics: innovative cancer therapies

Membrane contact sites (MCS) are specialized regions where organelles are closely interconnected through membrane structures, facilitating the transfer and exchange of ions, lipids, and other molecules. This proximity enables a synergistic regulation of cellular homeostasis and functions. The formation and maintenance of these contact sites are governed by specific proteins that bring organelle membranes into close apposition, thereby enabling functional crosstalk between cellular compartments. In eukaryotic cells, lipids are primarily synthesized and metabolized within distinct organelles and must be transported through MCS to ensure proper cellular function. Consequently, MCS act as pivotal platforms for lipid synthesis and trafficking, particularly in cancer cells and immune cells within the tumor microenvironment, where dynamic alterations are critical for maintaining lipid homeostasis. This article provides a comprehensive analysis of how these cells exploit membrane contact sites to modulate lipid synthesis, metabolism, and transport, with a specific focus on how MCS-mediated lipid dynamics influence tumor progression. We also examine the differences in MCS and associated molecules across various cancer types, exploring novel therapeutic strategies targeting MCS-related lipid metabolism for the development of anticancer drugs, while also addressing the challenges involved.

Packaging "vegetable oils": Insights into plant lipid droplet proteins

Plant neutral lipids, also known as "vegetable oils", are synthesized within the endoplasmic reticulum (ER) membrane and packaged into subcellular compartments called lipid droplets (LDs) for stable storage in the cytoplasm. The biogenesis, modulation, and degradation of cytoplasmic LDs in plant cells are orchestrated by a variety of proteins localized to the ER, LDs, and peroxisomes. Recent studies of these LD-related proteins have greatly advanced our understanding of LDs not only as steady oil depots in seeds but also as dynamic cell organelles involved in numerous physiological processes in different tissues and developmental stages of plants. In the past 2 decades, technology advances in proteomics, transcriptomics, genome sequencing, cellular imaging and protein structural modeling have markedly expanded the inventory of LD-related proteins, provided unprecedented structural and functional insights into the protein machinery modulating LDs in plant cells, and shed new light on the functions of LDs in nonseed plant tissues as well as in unicellular algae. Here, we review critical advances in revealing new LD proteins in various plant tissues, point out structural and mechanistic insights into key proteins in LD biogenesis and dynamic modulation, and discuss future perspectives on bridging our knowledge gaps in plant LD biology.

Lipid droplets in central nervous system and functional profiles of brain cells containing lipid droplets in various diseases

Lipid droplets (LDs), serving as the convergence point of energy metabolism and multiple signaling pathways, have garnered increasing attention in recent years. Different cell types within the central nervous system (CNS) can regulate energy metabolism to generate or degrade LDs in response to diverse pathological stimuli. This article provides a comprehensive review on the composition of LDs in CNS, their generation and degradation processes, their interaction mechanisms with mitochondria, the distribution among different cell types, and the roles played by these cells-particularly microglia and astrocytes-in various prevalent neurological disorders. Additionally, we also emphasize the paradoxical role of LDs in post-cerebral ischemia inflammation and explore potential underlying mechanisms, aiming to identify novel therapeutic targets for this disease.

Dynamical modelling of lipid droplet formation suggests a key function of membrane phospholipids

Cells store triacylglycerol (TAG) within lipid droplets (LDs). A dynamic model describing complete LD formation at the endoplasmic reticulum (ER) membrane does not yet exist. A biochemical-biophysical model of LD synthesis is proposed. It describes the time-dependent accumulation of TAG in the ER membrane as the formation of a potential LD (pLD) bounded by spherical caps of the inner and outer monolayers of the membrane. The expansion rate of the pLD depends on the TAG supply, the elastic properties of the ER membrane, and the recruitment of phospholipids (PLs) to the cap-covering monolayers. Model simulations provided the following insights: (a) Marginal differences in the surface tension of the cap monolayers are sufficient to fully drive the expansion of the pLD towards the cytosol or lumen. (b) Selective reduction of PL supply to the luminal monolayer ensures stable formation of cytosolic LDs, irrespective of variations in the elasto-mechanical properties of the ER membrane. (c) The rate of TAG supply to the cytosolic monolayer has a major effect on the size and maturation time of LDs but has no significant effect on the TAG export per individual LD. The recruitment of additional PLs to the cap monolayers of pLDs critically controls the budding direction, size, and maturation time of LDs. The ability of cells to acquire additional LD initiation sites appears to be key to coping with acutely high levels of potentially toxic free fatty acids.

Seipin governs phosphatidic acid homeostasis at the inner nuclear membrane

The nuclear envelope is a specialized subdomain of the endoplasmic reticulum and comprises the inner and outer nuclear membranes. Despite the crucial role of the inner nuclear membrane in genome regulation, its lipid metabolism remains poorly understood. Phosphatidic acid (PA) is essential for membrane growth as well as lipid storage. Using a genome-wide lipid biosensor screen in S. cerevisiae, we identify regulators of inner nuclear membrane PA homeostasis, including yeast Seipin, a known mediator of nuclear lipid droplet biogenesis. Here, we show that Seipin preserves nuclear envelope integrity by preventing its deformation and ectopic membrane formation. Mutations of specific regions of Seipin, some linked to human lipodystrophy, disrupt PA distribution at the inner nuclear membrane and nuclear lipid droplet formation. Investigating the Seipin co-factor Ldb16 reveals that a triacylglycerol binding site is crucial for lipid droplet formation, whereas PA regulation can be functionally separated. Our study highlights the potential of lipid biosensor screens for examining inner nuclear membrane lipid metabolism.

A novel bacterial effector protein mediates ER-LD membrane contacts to regulate host lipid droplets

Effective intracellular communication between cellular organelles occurs at dedicated membrane contact sites (MCSs). Tether proteins are responsible for the establishment of MCSs, enabling direct communication between organelles to ensure organelle function and host cell homeostasis. While recent research has identified tether proteins in several bacterial pathogens, their functions have predominantly been associated with mediating inter-organelle communication between the bacteria containing vacuole (BCV) and the host endoplasmic reticulum (ER). Here, we identify a novel bacterial effector protein, CbEPF1, which acts as a molecular tether beyond the confines of the BCV and facilitates interactions between host cell organelles. Coxiella burnetii, an obligate intracellular bacterial pathogen, encodes the FFAT motif-containing protein CbEPF1 which localizes to host lipid droplets (LDs). CbEPF1 establishes inter-organelle contact sites between host LDs and the ER through its interactions with VAP family proteins. Intriguingly, CbEPF1 modulates growth of host LDs in a FFAT motif-dependent manner. These findings highlight the potential for bacterial effector proteins to impact host cellular homeostasis by manipulating inter-organelle communication beyond conventional BCVs.

α/β hydrolase domain-containing protein 1 acts as a lysolipid lipase and is involved in lipid droplet formation

Lipid droplets (LDs) are the major sites of lipid and energy homeostasis. However, few LD biogenesis proteins have been identified. Using model microalga Chlamydomonas, we show that ABHD1, an α/β-hydrolase domain-containing protein, is localized to the LD surface and stimulates LD formation through two actions: one enzymatic and one structural. The knockout mutants contained similar amounts of triacylglycerols (TAG) but their LDs showed a higher content of lyso-derivatives of betaine lipid diacylglyceryl-N,N,N-trimethylhomoserine (DGTS). Over-expression of ABHD1 increased LD abundance and boosted TAG content. Purified recombinant ABHD1 hydrolyzed lyso-DGTS, producing a free fatty acid and a glyceryltrimethylhomoserine. In vitro droplet-embedded vesicles showed that ABHD1 promoted LD emergence. Taken together, these results identify ABHD1 as a new player in LD formation by its lipase activity on lyso-DGTS and by its distinct biophysical property. This study further suggests that lipases targeted to LDs and able to act on their polar lipid coat may be interesting tools to promote LD assembly in eukaryotic cells.

Cross-ancestry meta-genome-wide association studies provide insights to the understanding of semen traits in pigs

Semen traits play a crucial role in pig reproduction and fertility. However, limited data availability hinder a comprehensive understanding of the genetic mechanisms underlying these traits. In this study, we integrated 597 299 ejaculates and 3 596 sequence data to identify genetic variants and candidate genes related to four semen traits, including sperm progressive motility (MOT), semen volume, sperm concentration (CON), and effective sperm count (SUM). A cross-ancestry meta-genome-wide association study was conducted to detect 163 lead single nucleotide polymorphisms (SNPs) associated with MOT, CON, and SUM. Subsequently, transcriptome-wide association studies and colocalisation analyses were integrated to identify 176 candidate genes, many of which have documented roles in spermatogenesis or male mammal semen traits. Our analysis highlighted the potential involvement of CSM5, PDZD9, and LDAF1 in regulating semen traits through multiple methods. Finally, to validate the function of significant SNPs, we performed genomic feature best linear unbiased prediction in 348 independent pigs using identified trait-related SNP subsets as genomic features. We found that integrating the top 0.1, 1, and 5% significant SNPs as genomic features could enhance genomic prediction accuracy for CON and MOT compared to traditional genomic best linear unbiased prediction. This study contributes to a comprehensive understanding of the genetic mechanisms of boar semen traits and provides insight for developing genomic selection models.

The evolving landscape of ER-LD contact sites

Lipid droplets (LDs) are evolutionarily conserved dynamic organelles that play an important role in cellular physiology. Growing evidence suggests that LD biogenesis occurs at discrete endoplasmic reticulum (ER) subdomains demarcated by the lipodystrophy protein, Seipin, lack of which impairs adipogenesis. However, the mechanisms of how these domains are selected is not completely known. These ER sites undergo ordered assembly of proteins and lipids to initiate LD biogenesis and facilitate establishment of ER-LD contact sites, a prerequisite for proper growth and maturation of droplets. LDs retain both physical and functional association with the ER throughout their lifecycle to facilitate bi-directional communication, such as exchange of proteins and lipids between the two organelles at these ER-LD contact sites. In recent years several molecular tethers have been identified that bridge ER and LDs together including few proteins that are found exclusively at these ER-LD contact interface. Here, we discuss recent advances in understanding the role of factors that ensure functionality of ER-LD contact site machinery for LD homeostasis.

Mechanisms of lipid droplet degradation

Lipid droplets (LDs) are subcellular organelles that play an integral role in lipid metabolism by regulating the storage and release of fatty acids, which are essential for energy production and various cellular processes. Lipolysis and lipophagy are the two major LD degradation pathways that mediate the utilization of lipids stored in these organelles. Recent studies have further uncovered alternative pathways, including direct lysosomal LD degradation and LD exocytosis. Here, we highlight recent findings that dissect the molecular basis of these diverse LD degradation pathways. Then, we discuss speculations on the crosstalk among these pathways and the potential unconventional roles of LD degradation.

Lipid Droplets Big and Small: Basic Mechanisms That Make Them All

Lipid droplets (LDs) are dynamic storage organelles with central roles in lipid and energy metabolism. They consist of a core of neutral lipids, such as triacylglycerol, which is surrounded by a monolayer of phospholipids and specialized surface proteins. The surface composition determines many of the LD properties, such as size, subcellular distribution, and interaction with partner organelles. Considering the diverse energetic and metabolic demands of various cell types, it is not surprising that LDs are highly heterogeneous within and between cell types. Despite their diversity, all LDs share a common biogenesis mechanism. However, adipocytes have evolved specific adaptations of these basic mechanisms, enabling the regulation of lipid and energy metabolism at both the cellular and organismal levels. Here, we discuss recent advances in the understanding of both the general mechanisms of LD biogenesis and the adipocyte-specific adaptations controlling these fascinating organelles.

All-Atom Membrane Builder via Multiscale Simulation

I present an automated and flexible tool designed for constructing bilayer membranes at all-atom (AA) resolution. The builder initiates the construction and equilibration of bilayer membranes at Martini coarse-grained (CG) resolution, followed by resolution enhancement to the atomic level using the accompanying backmapping tool. Notably, this tool enables users to create bilayer membranes with user-defined lipid compositions and protein structures, while also offering the flexibility to accommodate new lipid types. To assess the simplicity and robustness of the tool, I demonstrate the construction of several membranes incorporating protein structures. The tool is freely available at github.com/ksy141/mstool.

Targeting lipid droplets and lipid droplet-associated proteins: a new perspective on natural compounds against metabolic diseases

BACKGROUND

Lipid droplet (LD) is a metabolically active organelle, which changes dynamically with the metabolic state and energy requirements of cells. Proteins that either insert into the LD phospholipid monolayer or are present in the cytoplasm, playing a crucial role in lipid homeostasis and signaling regulation, are known as LD-associated proteins.

METHODS

The keywords "lipid droplets" and "metabolic diseases" were used to obtain literature on LD metabolism and pathological mechanism. After searching databases including Scopus, OVID, Web of Science, and PubMed from 2013 to 2024 using terms like "lipid droplets", "lipid droplet-associated proteins", "fatty liver disease", "diabetes", "diabetic kidney disease", "obesity", "atherosclerosis", "hyperlipidemia", "natural drug monomers" and "natural compounds", the most common natural compounds were identified in about 954 articles. Eventually, a total of 91 studies of 10 natural compounds reporting in vitro or in vivo studies were refined and summarized.

RESULTS

The most frequently used natural compounds include Berberine, Mangostin, Capsaicin, Caffeine, Genistein, Epigallocatechin-3-gallate, Chlorogenic acid, Betaine, Ginsenoside, Resveratrol. These natural compounds interact with LD-associated proteins and help ameliorate abnormal LDs in various metabolic diseases.

CONCLUSION

Natural compounds involved in the regulation of LDs and LD-associated proteins hold promise for treating metabolic diseases. Further research into these interactions may lead to new therapeutic applications.

CHARMM-GUI Membrane Builder for Lipid Droplet Modeling and Simulation

Lipid droplets (LDs) are organelles that are necessary for eukaryotic and prokaryotic metabolism and energy storage. They have a unique structure consisting of a spherical phospholipid monolayer encasing neutral lipids such as triacylglycerol (TAG). LDs have garnered increased interest for their implications in disease and for drug delivery applications. Consequently, there is an increased need for tools to study their structure, composition, and dynamics in biological contexts. In this work, we utilize CHARMM-GUI Membrane Builder to simulate and analyze LDs with and without a plant LD protein, oleosin. The results show that Membrane Builder can generate biologically relevant all-atom LD systems with relatively short equilibration times using a new TAG library having optimized headgroup parameters. TAG molecules originally inserted into a lipid bilayer aggregate in the membrane center, forming a TAG-only core flanked by two monolayers. The TAG-only core thickness stably grows with increasing TAG mole fraction. A 70 % TAG system has a core that is thick enough to house oleosin without its interactions with the distal leaflet or disruption of its secondary structure. We hope that Membrane Builder can aid in the future study of LD systems, including their structure and dynamics with and without proteins.

The lipid droplet assembly complex consists of seipin and four accessory factors in budding yeast

Seipin, a crucial protein for cellular lipid droplet (LD) assembly, oligomerizes at the interface between the endoplasmic reticulum and LDs to facilitate neutral lipid packaging. Using proximity labeling, we identified four proteins-Ldo45, Ldo16, Tgl4, and Pln1-that are recruited to the vicinity of yeast seipin, the Sei1-Ldb16 complex, exclusively when seipin function is intact, hence termed seipin accessory factors. Localization studies identified Tgl4 at the endoplasmic reticulum-LD contact site, in contrast to Ldo45, Ldo16, and Pln1 at the LD surface. Cells with compromised seipin function resulted in uneven distribution of these proteins with aberrant LDs, supporting a central role of seipin in orchestrating their association with the LD. Overexpression of any seipin accessory factor causes LD aggregation and affects a subset of LD protein distribution, highlighting the importance of their stoichiometry. Although single factor mutations show minor LD morphology changes, the combined mutations have additive effects. Lastly, we present evidence that seipin accessory factors assemble and interact with seipin in the absence of neutral lipids and undergo dynamical rearrangements during LD formation induction, with Ldo45 acting as a central hub recruiting other factors to interact with the seipin complex.

A spatial expression atlas of the adult human proximal small intestine

The mouse small intestine shows profound variability in gene expression along the crypt-villus axis1,2. Whether similar spatial heterogeneity exists in the adult human gut remains unclear. Here we use spatial transcriptomics, spatial proteomics and single-molecule fluorescence in situ hybridization to reconstruct a comprehensive spatial expression atlas of the adult human proximal small intestine. We describe zonated expression and cell type representation for epithelial, mesenchymal and immune cell types. We find that migrating enterocytes switch from lipid droplet assembly and iron uptake at the villus bottom to chylomicron biosynthesis and iron release at the tip. Villus tip cells are pro-immunogenic, recruiting γδ T cells and macrophages to the tip, in contrast to their immunosuppressive roles in mouse. We also show that the human small intestine contains abundant serrated and branched villi that are enriched at the tops of circular folds. Our study presents a detailed resource for understanding the biology of the adult human small intestine.

Adipogenin Dictates Adipose Tissue Expansion by Facilitating the Assembly of a Dodecameric Seipin Complex

Adipogenin (Adig) is an evolutionarily conserved microprotein and is highly expressed in adipose tissues and testis. Here, we identify Adig as a critical regulator for lipid droplet formation in adipocytes. We determine that Adig interacts directly with seipin, leading to the formation of a rigid complex. We solve the structure of the seipin/Adig complex by Cryo-EM at 2.98Å overall resolution. Surprisingly, seipin can form two unique oligomers, undecamers and dodecamers. Adig selectively binds to the dodecameric seipin complex. We further find that Adig promotes seipin assembly by stabilizing and bridging adjacent seipin subunits. Functionally, Adig plays a key role in generating lipid droplets in adipocytes. In mice, inducible overexpression of Adig in adipocytes substantially increases fat mass, with enlarged lipid droplets. It also elevates thermogenesis during cold exposure. In contrast, inducible adipocyte-specific Adig knockout mice manifest aberrant lipid droplet formation in brown adipose tissues and impaired cold tolerance.

Review article: Hepatic steatosis and its associations with acute and chronic liver diseases

BACKGROUND

Hepatic steatosis is a common finding in liver histopathology and the hallmark of metabolic dysfunction-associated steatotic liver disease (MASLD), formerly known as non-alcoholic fatty liver disease (NAFLD), whose global prevalence is rising.

AIMS

To review the histopathology of hepatic steatosis and its mechanisms of development and to identify common and rare disease associations.

METHODS

We reviewed literature on the basic science of lipid droplet (LD) biology and clinical research on acute and chronic liver diseases associated with hepatic steatosis using the PubMed database.

RESULTS

A variety of genetic and environmental factors contribute to the development of chronic hepatic steatosis or steatotic liver disease, which typically appears macrovesicular. Microvesicular steatosis is associated with acute mitochondrial dysfunction and liver failure. Fat metabolic processes in hepatocytes whose dysregulation leads to the development of steatosis include secretion of lipoprotein particles, uptake of remnant lipoprotein particles or free fatty acids from blood, de novo lipogenesis, oxidation of fatty acids, lipolysis and lipophagy. Hepatic insulin resistance is a key feature of MASLD. Seipin is a polyfunctional protein that facilitates LD biogenesis. Assembly of hepatitis C virus takes place on LD surfaces. LDs make important, functional contact with the endoplasmic reticulum and other organelles.

CONCLUSIONS

Diverse liver pathologies are associated with hepatic steatosis, with MASLD being the most important contributor. The biogenesis and dynamics of LDs in hepatocytes are complex and warrant further investigation. Organellar interfaces permit co-regulation of lipid metabolism to match generation of potentially toxic lipid species with their LD depot storage.

Giant organelle vesicles to uncover intracellular membrane mechanics and plasticity

Tools for accessing and studying organelles remain underdeveloped. Here, we present a method by which giant organelle vesicles (GOVs) are generated by submitting cells to a hypotonic medium followed by plasma membrane breakage. By this means, GOVs ranging from 3 to over 10 µm become available for micromanipulation. GOVs are made from organelles such as the endoplasmic reticulum, endosomes, lysosomes and mitochondria, or in contact with one another such as giant mitochondria-associated ER membrane vesicles. We measure the mechanical properties of each organelle-derived GOV and find that they have distinct properties. In GOVs procured from Cos7 cells, for example, bending rigidities tend to increase from the endoplasmic reticulum to the plasma membrane. We also found that the mechanical properties of giant endoplasmic reticulum vesicles (GERVs) vary depending on their interactions with other organelles or the metabolic state of the cell. Lastly, we demonstrate GERVs' biochemical activity through their capacity to synthesize triglycerides and assemble lipid droplets. These findings underscore the potential of GOVs as valuable tools for studying the biophysics and biology of organelles.

Molecular dynamics simulations of intracellular lipid droplets: a new tool in the toolbox

Lipid droplets (LDs) are ubiquitous intracellular organelles with a central role in multiple lipid metabolic pathways. However, identifying correlations between their structural properties and their biological activity has proved challenging, owing to their unique physicochemical properties as compared with other cellular membranes. In recent years, molecular dynamics (MD) simulations, a computational methodology allowing the accurate description of molecular assemblies down to their individual components, have been demonstrated to be a useful and powerful approach for studying LD structural and dynamical properties. In this short review, we attempt to highlight, as comprehensively as possible, how MD simulations have contributed to our current understanding of multiple molecular mechanisms involved in LD biology.

CIDE proteins and their regulatory mechanisms in lipid droplet fusion and growth

The cell death-inducing DFF45-like effector (CIDE) proteins, including Cidea, Cideb, and Cidec/Fsp27, regulate various aspects of lipid homeostasis, including lipid storage, lipolysis, and lipid secretion. This review focuses on the physiological roles of CIDE proteins based on studies on knockout mouse models and human patients bearing CIDE mutations. The primary cellular function of CIDE proteins is to localize to lipid droplets (LDs) and to control LD fusion and growth across different cell types. We propose a four-step process of LD fusion, characterized by (a) the recruitment of CIDE proteins to the LD surface and CIDE movement, (b) the enrichment and condensate formation of CIDE proteins to form LD fusion plates at LD-LD contact sites, (c) lipid transfer through lipid-permeable passageways within the fusion plates, and (d) the completion of LD fusion. Lastly, we outline CIDE-interacting proteins as regulatory factors, as well as their contribution in LD fusion.

A unifying mechanism for seipin-mediated lipid droplet formation

Lipid droplets (LDs) are dynamic organelles essential for cellular lipid homeostasis. Assembly of LDs occurs in the endoplasmic reticulum (ER), and the conserved ER membrane protein seipin emerged as a key player in this process. Here, we review recent advances provided by structural, biochemical, and in silico analysis that revealed mechanistic insights into the molecular role of the seipin complexes and led to an updated model for LD biogenesis. We further discuss how other ER components cooperate with seipin during LD biogenesis. Understanding the molecular mechanisms underlying seipin-mediated LD assembly is important to uncover the fundamental aspects of lipid homeostasis and organelle biogenesis and to provide hints on the pathogenesis of lipid storage disorders.

Molecular mechanisms of perilipin protein function in lipid droplet metabolism

Perilipins are abundant lipid droplet (LD) proteins present in all metazoans and also in Amoebozoa and fungi. Humans express five perilipins, which share a similar domain organization: an amino-terminal PAT domain and an 11-mer repeat region, which can fold into amphipathic helices that interact with LDs, followed by a structured carboxy-terminal domain. Variations of this organization that arose during vertebrate evolution allow for functional specialization between perilipins in relation to the metabolic needs of different tissues. We discuss how different features of perilipins influence their interaction with LDs and their cellular targeting. PLIN1 and PLIN5 play a direct role in lipolysis by regulating the recruitment of lipases to LDs and LD interaction with mitochondria. Other perilipins, particularly PLIN2, appear to protect LDs from lipolysis, but the molecular mechanism is not clear. PLIN4 stands out with its long repetitive region, whereas PLIN3 is most widely expressed and is used as a nascent LD marker. Finally, we discuss the genetic variability in perilipins in connection with metabolic disease, prominent for PLIN1 and PLIN4, underlying the importance of understanding the molecular function of perilipins.

LDO proteins and Vac8 form a vacuole-lipid droplet contact site to enable starvation-induced lipophagy in yeast

Lipid droplets (LDs) are fat storage organelles critical for energy and lipid metabolism. Upon nutrient exhaustion, cells consume LDs via gradual lipolysis or via lipophagy, the en bloc uptake of LDs into the vacuole. Here, we show that LDs dock to the vacuolar membrane via a contact site that is required for lipophagy in yeast. The LD-localized LDO proteins carry an intrinsically disordered region that directly binds vacuolar Vac8 to form vCLIP, the vacuolar-LD contact site. Nutrient limitation drives vCLIP formation, and its inactivation blocks lipophagy, resulting in impaired caloric restriction-induced longevity. We establish a functional link between lipophagy and microautophagy of the nucleus, both requiring Vac8 to form respective contact sites upon metabolic stress. In sum, we identify the tethering machinery of vCLIP and find that Vac8 provides a platform for multiple and competing contact sites associated with autophagy.

A metabolically controlled contact site between vacuoles and lipid droplets in yeast

The lipid droplet (LD) organization proteins Ldo16 and Ldo45 affect multiple aspects of LD biology in yeast. They are linked to the LD biogenesis machinery seipin, and their loss causes defects in LD positioning, protein targeting, and breakdown. However, their molecular roles remained enigmatic. Here, we report that Ldo16/45 form a tether complex with Vac8 to create vacuole lipid droplet (vCLIP) contact sites, which can form in the absence of seipin. The phosphatidylinositol transfer protein (PITP) Pdr16 is a further vCLIP-resident recruited specifically by Ldo45. While only an LD subpopulation is engaged in vCLIPs at glucose-replete conditions, nutrient deprivation results in vCLIP expansion, and vCLIP defects impair lipophagy upon prolonged starvation. In summary, Ldo16/45 are multifunctional proteins that control the formation of a metabolically regulated contact site. Our studies suggest a link between LD biogenesis and breakdown and contribute to a deeper understanding of how lipid homeostasis is maintained during metabolic challenges.

Lipid- and phospho-regulation of CTP:Phosphocholine Cytidylyltransferase α association with nuclear lipid droplets

Fatty acids stored in triacylglycerol-rich lipid droplets are assembled with a surface monolayer composed primarily of phosphatidylcholine (PC). Fatty acids stimulate PC synthesis by translocating CTP:phosphocholine cytidylyltransferase (CCT) α to the inner nuclear membrane, nuclear lipid droplets (nLD) and lipid associated promyelocytic leukemia (PML) structures (LAPS). Huh7 cells were used to identify how CCTα translocation onto these nuclear structures are regulated by fatty acids and phosphorylation of its serine-rich P-domain. Oleate treatment of Huh7 cells increased nLDs and LAPS that became progressively enriched in CCTα. In cells expressing the phosphatidic acid phosphatase Lipin1α or 1β, the expanded pool of nLDs and LAPS had a proportional increase in associated CCTα. In contrast, palmitate induced few nLDs and LAPS and inhibited the oleate-dependent translocation of CCTα without affecting total nLDs. Phospho-memetic or phospho-null mutations in the P-domain revealed that a 70% phosphorylation threshold, rather than site-specific phosphorylation, regulated CCTα association with nLDs and LAPS. In vitro candidate kinase and inhibitor studies in Huh7 cells identified cyclin-dependent kinase (CDK) 1 and 2 as putative P-domain kinases. In conclusion, CCTα translocation onto nLDs and LAPS is dependent on available surface area and fatty acid composition, as well as threshold phosphorylation of the P-domain potentially involving CDKs.

Lipid droplets and cellular lipid flux

Lipid droplets are dynamic organelles that store neutral lipids, serve the metabolic needs of cells, and sequester lipids to prevent lipotoxicity and membrane damage. Here we review the current understanding of the mechanisms of lipid droplet biogenesis and turnover, the transfer of lipids and metabolites at membrane contact sites, and the role of lipid droplets in regulating fatty acid flux in lipotoxicity and cell death.

Lipid droplets, autophagy, and ageing: A cell-specific tale

Lipid droplets are the essential organelle for storing lipids in a cell. Within the variety of the human body, different cells store, utilize and release lipids in different ways, depending on their intrinsic function. However, these differences are not well characterized and, especially in the context of ageing, represent a key factor for cardiometabolic diseases. Whole body lipid homeostasis is a central interest in the field of cardiometabolic diseases. In this review we characterize lipid droplets and their utilization via autophagy and describe their diverse fate in three cells types central in cardiometabolic dysfunctions: adipocytes, hepatocytes, and macrophages.

ER-LD Membrane Contact Sites: A Budding Area in the Pathogen Survival Strategy

The endoplasmic reticulum (ER) and lipid droplets (LDs) are essential organelles involved in lipid synthesis, storage, and transport. Physical membrane contacts between the ER and LDs facilitate lipid and protein exchange and thus play a critical role in regulating cellular lipid homeostasis. Recent research has revealed that ER-LD membrane contact sites are targeted by pathogens seeking to exploit host lipid metabolic processes. Both viruses and bacteria manipulate ER-LD membrane contact sites to enhance their replication and survival within the host. This review discusses the research advancements elucidating the mechanisms by which pathogens manipulate the ER-LD contacts through protein molecular mimicry and host cell protein manipulation, thereby hijacking host lipid metabolic processes to facilitate pathogenesis. Understanding the crosstalk between ER and LDs during infection provides deeper insight into host lipid regulation and uncovers potential therapeutic targets for treating infectious diseases.

Contact-FP: A Dimerization-Dependent Fluorescent Protein Toolkit for Visualizing Membrane Contact Site Dynamics

Membrane contact sites (MCSs) are sites of close apposition between two organelles used to exchange ions, lipids, and information. Cells respond to changing environmental or developmental conditions by modulating the number, extent, or duration of MCSs. Because of their small size and dynamic nature, tools to study the dynamics of MCSs in live cells have been limited. Dimerization-dependent fluorescent proteins (ddFPs) targeted to organelle membranes are an ideal tool for studying MCS dynamics because they reversibly interact to fluoresce specifically at the interface between two organelles. Here, we build on previous work using ddFPs as sensors to visualize the morphology and dynamics of MCSs. We engineered a suite of ddFPs called Contact-FP that targets ddFP monomers to lipid droplets (LDs), the endoplasmic reticulum (ER), mitochondria, peroxisomes, lysosomes, plasma membrane, caveolae, and the cytoplasm. We show that these probes correctly localize to their target organelles. Using LDs as a test case, we demonstrate that Contact-FP pairs specifically localize to the interface between two target organelles. Titration of LD-mitochondria ddFPs revealed that these sensors can be used at high concentrations to drive MCSs or can be titrated down to minimally perturb and visualize endogenous MCSs. We show that Contact-FP probes can be used to: (1) visualize LD-mitochondria MCS dynamics, (2) observe changes in LD-mitochondria MCS dynamics upon overexpression of PLIN5, a known LD-mitochondrial tether, and (3) visualize two MCSs that share one organelle simultaneously (e.g., LD-mitochondria and LD-ER MCSs). Contact-FP probes can be optimized to visualize MCSs between any pair of organelles represented in the toolkit.

The Vacuole Lipid Droplet Contact Site vCLIP

Lipid droplets frequently form contact sites with the membrane of the vacuole, the lysosome-like organelle in yeast. These vacuole lipid droplet (vCLIP) contact sites respond strongly to metabolic cues: while only a subset of lipid droplets is bound to the vacuole when nutrients are abundant, other metabolic states induce stronger contact site formation. Physical lipid droplet-vacuole binding is related to the process of lipophagy, a lipid droplet-specific form of microautophagy. The molecular basis for the formation and function of vCLIP contact sites remained enigmatic for a long time. This knowledge gap was filled when it was found that vCLIP is formed by the structurally related lipid droplet tether proteins Ldo16 and Ldo45, and the vacuolar surface protein Vac8. Ldo45 additionally recruits the phosphatidylinositol transfer protein Pdr16 to vCLIP. Here, we review the literature on the lipid droplet-vacuole contact site in light of the progress in our understanding of its molecular basis and discuss future directions for the field.

Lipid droplet biogenesis in the ovary

Background

Lipid droplets (LDs) are organelles consisting of a central core of neutral lipids covered by a single layer of phospholipids and are found in most eukaryotic cells. Accumulating evidence suggests that LDs not only store neutral lipids but also coordinate with other organelles for lipid metabolism within cells.

Methods

This review focuses on the synthesis of LDs during follicular development and highlights the factors involved in the regulation of LD biogenesis within the ovary.

Main Findings

In the mammalian ovary, the presence of LDs has long been recognized mainly by morphological analysis. However, their distribution in the ovary varies according to the region and cell type; for example, LDs are abundant in the medulla, which has a rich blood vessel network, in interstitial cells, which are the site of steroid production, and surrounding growing follicles, while they are poor in granulosa cells within follicles. LDs are also enriched in the corpus luteum after ovulation and massively accumulate in atretic follicles during follicular growth. Furthermore, LD synthesis is synchronized with angiogenesis during follicular development.

Conclusion

Addressing the functional link between LD biogenesis and angiogenesis is essential for understanding the molecular basis underlying LD biology, as well as the ovarian dysfunction with metabolic disorders.

Liposome-Mediated Anti-Viral Drug Delivery Across Blood-Brain Barrier: Can Lipid Droplet Target Be Game Changers?

Lipid droplets (LDs) are subcellular organelles secreted from the endoplasmic reticulum (ER) that play a major role in lipid homeostasis. Recent research elucidates additional roles of LDs in cellular bioenergetics and innate immunity. LDs activate signaling cascades for interferon response and secretion of pro-inflammatory cytokines. Since balanced lipid homeostasis is critical for neuronal health, LDs play a crucial role in neurodegenerative diseases. RNA viruses enhance the secretion of LDs to support various phases of their life cycle in neurons which further leads to neurodegeneration. Targeting the excess LD formation in the brain could give us a new arsenal of antiviral therapeutics against neuroviruses. Liposomes are a suitable drug delivery system that could be used for drug delivery in the brain by crossing the Blood-Brain Barrier. Utilizing this, various pharmacological inhibitors and non-coding RNAs can be delivered that could inhibit the biogenesis of LDs or reduce their sizes, reversing the excess lipid-related imbalance in neurons. Liposome-Mediated Antiviral Drug Delivery Across Blood-Brain Barrier. Developing effective antiviral drug is challenging and it doubles against neuroviruses that needs delivery across the Blood-Brain Barrier (BBB). Lipid Droplets (LDs) are interesting targets for developing antivirals, hence targeting LD formation by drugs delivered using Liposomes can be game changers.

Rotavirus-mediated DGAT1 degradation: A pathophysiological mechanism of viral-induced malabsorptive diarrhea

Gastroenteritis is among the leading causes of mortality globally in infants and young children, with rotavirus (RV) causing ~258 million episodes of diarrhea and ~128,000 deaths annually in infants and children. RV-induced mechanisms that result in diarrhea are not completely understood, but malabsorption is a contributing factor. RV alters cellular lipid metabolism by inducing lipid droplet (LD) formation as a platform for replication factories named viroplasms. A link between LD formation and gastroenteritis has not been identified. We found that diacylglycerol O-acyltransferase 1 (DGAT1), the terminal step in triacylglycerol synthesis required for LD biogenesis, is degraded in RV-infected cells by a proteasome-mediated mechanism. RV-infected DGAT1-silenced cells show earlier and increased numbers of LD-associated viroplasms per cell that translate into a fourfold-to-fivefold increase in viral yield (P < 0.05). Interestingly, DGAT1 deficiency in children is associated with diarrhea due to altered trafficking of key ion transporters to the apical brush border of enterocytes. Confocal microscopy and immunoblot analyses of RV-infected cells and DGAT1-/- human intestinal enteroids (HIEs) show a decrease in expression of nutrient transporters, ion transporters, tight junctional proteins, and cytoskeletal proteins. Increased phospho-eIF2α (eukaryotic initiation factor 2 alpha) in DGAT1-/- HIEs, and RV-infected cells, indicates a mechanism for malabsorptive diarrhea, namely inhibition of translation of cellular proteins critical for nutrient digestion and intestinal absorption. Our study elucidates a pathophysiological mechanism of RV-induced DGAT1 deficiency by protein degradation that mediates malabsorptive diarrhea, as well as a role for lipid metabolism, in the pathogenesis of gastroenteritis.

Behavior of intracellular lipid droplets during cell division in HuH7 hepatoma cells

Intracellular lipid droplets (LDs) are ubiquitous organelles found in many cell types. During mitosis, membranous organelles, including mitochondria, are divided into small pieces and transferred to daughter cells; however, the process of LD transfer to daughter cells is not fully elucidated. Herein, we investigated the behavior of LDs during mitosis in HuH7 human hepatoma cells. While fragments of the Golgi apparatus were scattered in the cytosol during mitosis, intracellular LDs retained their size and spherical morphology as they translocated to the two daughter cells. LDs were initially distributed throughout the cell during prophase but positioned outside the spindle in metaphase, aligning at the far sides of the centrioles. A similar distribution of LDs during mitosis was observed in another hepatocarcinoma HepG2 cells. When the spindle was disrupted by nocodazole treatment or never in mitosis gene A-related kinase 2A knockdown, LDs were localized in the area outside the chromosomes, suggesting that spindle formation is not necessary for LD localization at metaphase. The amount of major LD protein perilipin 2 reduced while LDs were enriched in perilipin 3 during mitosis, indicating the potential alteration of LD protein composition. Conclusively, the behavior of LDs during mitosis is distinct from that of other organelles in hepatocytes.

The novel bacterial effector protein CbEPF1 mediates ER-LD membrane contacts to regulate host lipid droplet metabolism

Effective intracellular communication between cellular organelles is pivotal for maintaining cellular homeostasis. Tether proteins, which are responsible for establishing membrane contact sites between cell organelles, enable direct communication between organelles and ultimately influence organelle function and host cell homeostasis. While recent research has identified tether proteins in several bacterial pathogens, their functions have predominantly been associated with mediating inter-organelle communication specifically between the bacteria containing vacuole (BCV) and the host endoplasmic reticulum (ER). However, this study reveals a novel bacterial effector protein, CbEPF1, which acts as a molecular tether beyond the confines of the BCV and facilitates interactions between host cell organelles. Coxiella burnetii, an obligate intracellular bacterial pathogen, encodes the FFAT motif-containing protein CbEPF1 which localizes to host lipid droplets (LDs). CbEPF1 establishes inter-organelle contact sites between host LDs and the ER through its interactions with VAP family proteins. Intriguingly, CbEPF1 modulates growth of host LDs in a FFAT motif-dependent manner. These findings highlight the potential for bacterial effector proteins to impact host cellular homeostasis by manipulating inter-organelle communication beyond conventional BCVs.

Genetically encoded multimeric tags for subcellular protein localization in cryo-EM

Cryo-electron tomography (cryo-ET) allows for label-free high-resolution imaging of macromolecular assemblies in their native cellular context. However, the localization of macromolecules of interest in tomographic volumes can be challenging. Here we present a ligand-inducible labeling strategy for intracellular proteins based on fluorescent, 25-nm-sized, genetically encoded multimeric particles (GEMs). The particles exhibit recognizable structural signatures, enabling their automated detection in cryo-ET data by convolutional neural networks. The coupling of GEMs to green fluorescent protein-tagged macromolecules of interest is triggered by addition of a small-molecule ligand, allowing for time-controlled labeling to minimize disturbance to native protein function. We demonstrate the applicability of GEMs for subcellular-level localization of endogenous and overexpressed proteins across different organelles in human cells using cryo-correlative fluorescence and cryo-ET imaging. We describe means for quantifying labeling specificity and efficiency, and for systematic optimization for rare and abundant protein targets, with emphasis on assessing the potential effects of labeling on protein function.

Concept of lipid droplet biogenesis

Lipid droplets (LD) are functionally conserved fat storage organelles found in all cell types. LDs have a unique structure comprising of a hydrophobic core of neutral lipids (fat), triacylglycerol (TAG) and cholesterol esters (CE) surrounded by a phospholipid monolayer. LD surface is decorated by a multitude of proteins and enzymes rendering this compartment functional. Accumulating evidence suggests that LDs originate from discrete ER-subdomains, demarcated by the lipodystrophy protein seipin, however, the mechanisms of which are not well understood. LD biogenesis factors together with biophysical properties of the ER membrane orchestrate spatiotemporal regulation of LD nucleation and growth at specific ER subdomains in response to metabolic cues. Defects in LD formation manifests in several human pathologies, including obesity, lipodystrophy, ectopic fat accumulation, and insulin resistance. Here, we review recent advances in understanding the molecular events during initial stages of eukaryotic LD assembly and discuss the critical role of factors that ensure fidelity of this process.

GDF11 slows excitatory neuronal senescence and brain ageing by repressing p21

As a major neuron type in the brain, the excitatory neuron (EN) regulates the lifespan in C. elegans. How the EN acquires senescence, however, is unknown. Here, we show that growth differentiation factor 11 (GDF11) is predominantly expressed in the EN in the adult mouse, marmoset and human brain. In mice, selective knock-out of GDF11 in the post-mitotic EN shapes the brain ageing-related transcriptional profile, induces EN senescence and hyperexcitability, prunes their dendrites, impedes their synaptic input, impairs object recognition memory and shortens the lifespan, establishing a functional link between GDF11, brain ageing and cognition. In vitro GDF11 deletion causes cellular senescence in Neuro-2a cells. Mechanistically, GDF11 deletion induces neuronal senescence via Smad2-induced transcription of the pro-senescence factor p21. This work indicates that endogenous GDF11 acts as a brake on EN senescence and brain ageing.

Identification of an alternative triglyceride biosynthesis pathway

Triacylglycerols (TAGs) are the main source of stored energy in the body, providing an important substrate pool for mitochondrial beta-oxidation. Imbalances in the amount of TAGs are associated with obesity, cardiac disease and various other pathologies1,2. In humans, TAGs are synthesized from excess, coenzyme A-conjugated fatty acids by diacylglycerol O-acyltransferases (DGAT1 and DGAT2)3. In other organisms, this activity is complemented by additional enzymes4, but whether such alternative pathways exist in humans remains unknown. Here we disrupt the DGAT pathway in haploid human cells and use iterative genetics to reveal an unrelated TAG-synthesizing system composed of a protein we called DIESL (also known as TMEM68, an acyltransferase of previously unknown function) and its regulator TMX1. Mechanistically, TMX1 binds to and controls DIESL at the endoplasmic reticulum, and loss of TMX1 leads to the unconstrained formation of DIESL-dependent lipid droplets. DIESL is an autonomous TAG synthase, and expression of human DIESL in Escherichia coli endows this organism with the ability to synthesize TAG. Although both DIESL and the DGATs function as diacylglycerol acyltransferases, they contribute to the cellular TAG pool under specific conditions. Functionally, DIESL synthesizes TAG at the expense of membrane phospholipids and maintains mitochondrial function during periods of extracellular lipid starvation. In mice, DIESL deficiency impedes rapid postnatal growth and affects energy homeostasis during changes in nutrient availability. We have therefore identified an alternative TAG biosynthetic pathway driven by DIESL under potent control by TMX1.

Defensive-lipid droplets: Cellular organelles designed for antimicrobial immunity

Microbes have developed many strategies to subvert host organisms, which, in turn, evolved several innate immune responses. As major lipid storage organelles of eukaryotes, lipid droplets (LDs) are an attractive source of nutrients for invaders. Intracellular viruses, bacteria, and protozoan parasites induce and physically interact with LDs, and the current view is that they "hijack" LDs to draw on substrates for host colonization. This dogma has been challenged by the recent demonstration that LDs are endowed with a protein-mediated antibiotic activity, which is upregulated in response to danger signals and sepsis. Dependence on host nutrients could be a generic "Achilles' heel" of intracellular pathogens and LDs a suitable chokepoint harnessed by innate immunity to organize a front-line defense. Here, we will provide a brief overview of the state of the conflict and discuss potential mechanisms driving the formation of the 'defensive-LDs' functioning as hubs of innate immunity.

ATGL-mediated lipophagy balances cholesterol-induced inflammation in pathogen infected Apostichopus japonicus coelomocytes

Cholesterol metabolism can be dynamically altered in response to pathogen infection that ensure proper macrophage inflammatory function in mammals. However, it is unclear whether the dynamic between cholesterol accumulation and breakdown could induce or suppress inflammation in aquatic animal. Here, we aimed to investigate the cholesterol metabolic response to LPS stimulation in coelomocytes of Apostichopus japonicus, and to elucidate the mechanism of lipophagy in regulating cholesterol-related inflammation. LPS stimulation significantly increased intracellular cholesterol levels at early time point (12 h), and the increase in cholesterol levels is associated with AjIL-17 upregulation. Excessive cholesterol in coelomocytes of A. japonicus was rapidly converted to cholesteryl esters (CEs) and stored in lipid droplets (LDs) after 12 h of LPS stimulation and prolonged for 18 h. Then, increased colocalization of LDs with lysosomes was observed at late time point of LPS treatment (24 h), accompanied by elevated expression of AjLC3 and decreased expression of Ajp62. At the same time, the expression of AjABCA1 rapidly increased, suggesting lipophagy induction. Moreover, we demonstrated that AjATGL is required for induction of lipophagy. Inducing lipophagy by AjATGL overexpression attenuated cholesterol-induced AjIL-17 expression. Overall, our study provides evidence that cholesterol metabolic response occurs upon LPS stimulation, which is actively involved in regulating the inflammatory response of coelomocytes. AjATGL-mediated lipophagy is responsible for cholesterol hydrolysis, thereby balancing cholesterol-induced inflammation in the coelomocytes of A. japonicus.

Regulation of lipid droplets and cholesterol metabolism in adrenal cortical cells

The adrenal gland is composed of two distinctly different endocrine moieties. The interior medulla consists of neuroendocrine chromaffin cells that secrete catecholamines like adrenaline and noradrenaline, while the exterior cortex consists of steroidogenic cortical cells that produce steroid hormones, such as mineralocorticoids (aldosterone), glucocorticoids (cortisone and cortisol) and androgens. Synthesis of steroid hormones in cortical cells requires substantial amounts of cholesterol, which is the common precursor for steroidogenesis. Cortical cells may acquire cholesterol from de novo synthesis and uptake from circulating low- and high-density lipoprotein particles (LDL and HDL). As cholesterol is part of the plasma membrane in all mammalian cells and an important regulator of membrane fluidity, cellular levels of free cholesterol are tightly regulated. To ensure a robust supply of cholesterol for steroidogenesis and to avoid cholesterol toxicity, cortical cells store large amounts of cholesterol as cholesteryl esters in intracellular lipid droplets. Cortical steroidogenesis relies on both mobilization of cholesterol from lipid droplets and constant uptake of circulating cholesterol to replenish lipid droplet stores. This chapter will describe mechanisms involved in cholesterol uptake, cholesteryl ester synthesis, lipid droplet formation, hydrolysis of stored cholesteryl esters, as well as their impact on steroidogenesis. Additionally, animal models and human diseases characterized by altered cortical cholesteryl ester storage, with or without abnormal steroidogenesis, will be discussed.

Identification of host regulators of Mycobacterium tuberculosis phenotypes uncovers a role for the MMGT1-GPR156 lipid droplet axis in persistence

The ability of Mycobacterium tuberculosis (Mtb) to establish latency affects disease and response to treatment. The host factors that influence the establishment of latency remain elusive. We engineered a multi-fluorescent Mtb strain that reports survival, active replication, and stressed non-replication states and determined the host transcriptome of the infected macrophages in these states. Additionally, we conducted a genome-wide CRISPR screen to identify host factors that modulated the phenotypic state of Mtb. We validated hits in a phenotype-specific manner and prioritized membrane magnesium transporter 1 (MMGT1) for a detailed mechanistic investigation. Mtb infection of MMGT1-deficient macrophages promoted a switch to persistence, upregulated lipid metabolism genes, and accumulated lipid droplets during infection. Targeting triacylglycerol synthesis reduced both droplet formation and Mtb persistence. The orphan G protein-coupled receptor GPR156 is a key inducer of droplet accumulation in ΔMMGT1 cells. Our work uncovers the role of MMGT1-GPR156-lipid droplets in the induction of Mtb persistence.

Structural insights into perilipin 3 membrane association in response to diacylglycerol accumulation

Lipid droplets (LDs) are dynamic organelles that contain an oil core mainly composed of triglycerides (TAG) that is surrounded by a phospholipid monolayer and LD-associated proteins called perilipins (PLINs). During LD biogenesis, perilipin 3 (PLIN3) is recruited to nascent LDs as they emerge from the endoplasmic reticulum. Here, we analyze how lipid composition affects PLIN3 recruitment to membrane bilayers and LDs, and the structural changes that occur upon membrane binding. We find that the TAG precursors phosphatidic acid and diacylglycerol (DAG) recruit PLIN3 to membrane bilayers and define an expanded Perilipin-ADRP-Tip47 (PAT) domain that preferentially binds DAG-enriched membranes. Membrane binding induces a disorder to order transition of alpha helices within the PAT domain and 11-mer repeats, with intramolecular distance measurements consistent with the expanded PAT domain adopting a folded but dynamic structure upon membrane binding. In cells, PLIN3 is recruited to DAG-enriched ER membranes, and this requires both the PAT domain and 11-mer repeats. This provides molecular details of PLIN3 recruitment to nascent LDs and identifies a function of the PAT domain of PLIN3 in DAG binding.